This invention relates to semiconductor memory devices.
More particularly, the present invention relates to an improved conductive interconnect for supplying currents to semiconductor random access memory devices that utilize a magnetic field.
A magnetic memory device has a structure which includes ferromagnetic layers separated by a non-magnetic layer. The ferromagnetic layers have a free magnetic moment vector that can be oriented in one of several preferred directions relative to a pinned magnetic moment vector that is fixed in direction. The orientation of the free magnetic moment vector relative to the pinned magnetic moment vector creates unique resistance states that are used to represent stored information. Accordingly, a detection of changes in resistance allows a magnetic memory device to provide stored information. In typical magnetic memory devices, two resistance states are available. The stored states can be read by passing a sense current through the cell in a sense interconnect because of the difference between the magnetic resistances of the states.
In magnetoresistive random access memory (hereinafter referred to as MRAM) devices, the memory cells are programmed by magnetic fields induced by a current carrying conductor such as a copper interconnect. Typically, two conductive orthogonal interconnects are employed, with one positioned above (hereinafter referred to as the bit interconnect) the MRAM device and the second positioned below (hereinafter referred to as the digit interconnect) the MRAM device. The purpose of the bit and digit interconnects is to provide magnetic fields for programming the MRAM device.
A ferromagnetic cladding layer can be positioned on each of the conductive interconnects to significantly increase the magnitude of the magnetic field and to control its direction. The use of ferromagnetic cladding layers enables the MRAM device to be used in low power applications by reducing the magnitudes of currents required to create a given magnetic field. Additionally, the ferromagnetic cladding layers can focus the magnetic fields in a desired direction to shield adjacent MRAM devices within an MRAM device array, and, therefore, to prevent inadvertently changing the memory states of the adjacent MRAM devices. More information as to cladding a MRAM device can be found in U.S. Pat. No. 6,211,090 entitled xe2x80x9cMethod of Fabricating Flux Concentrating Layer for use with Magnetoresistive Random Access Memoriesxe2x80x9d issued on Apr. 3, 2001.
One problem with using ferromagnetic cladding layers, however, is the creation of domains when high currents are passed through the conductive interconnect it partially surrounds (See FIG. 1). These domains create difficult to control remanent fields when the current in the conductive interconnect is reduced to zero and also create hysteresis in the ferromagnetic cladding response when further currents are applied.
For example, FIG. 1 illustrates a micromagnetic simulation of a single ferromagnetic cladding layer 1 which has a first side, a bottom, and a second side. It will be understood that the first side, bottom, and second side of ferromagnetic cladding layer 1 are all illustrated in the same plane for simplicity. Further, for ease of discussion, directions will be referenced to x-y coordinate system 48. The micromagnetic simulation calculates the orientation of a plurality of magnetic moment vectors 2 over a length of ferromagnetic cladding layer 1 and shows the formation of a domain 12 in the remanent state after ferromagnetic cladding layer 1 is saturated. Further, it is assumed that ferromagnetic cladding layer 1 has a thickness of 2.5 nm and includes NiFeCo.
Ideally, the plurality of magnetic moment vectors 2 would be aligned in the same direction and be oriented either parallel or anti-parallel to the length of ferromagnetic cladding layer 1 (e.g. the y-direction). However, the formation of domain 12 alters the alignment of plurality of magnetic moment vectors 2 proximate to domain 12, and, therefore, creates an induced positive charge 4 at a boundary 19 and an induced positive charge 3 at a boundary 18. Induced positive charge 3 and 4 create stray magnetic fields that can interact with the MRAM device.
Domain formation also causes hysteresis in the ferromagnetic cladding layer when subsequent currents are applied. Hysteresis limits the magnetic field response of the ferromagnetic cladding layer, and, therefore, reduces the performance of the ferromagnetic cladding layer and its ability to focus a given magnetic field at the MRAM device for a given current supplied. Thus, hysteresis can produce significant errors during the operation of the MRAM device.
FIG. 2 illustrates a plot 6 showing the change in magnetoresistance (e.g. magnetoresistance response) of a MRAM device from a magnetic field generated by a ferromagnetic cladded conductive interconnect. In this example, the ferromagnetic cladded conductive interconnect is oriented to produce a hard axis magnetic field to the MRAM device. From plot 6, as the current in the ferromagnetic cladded conductive interconnect is increased, the magnetoresistance response becomes non-linear and the affect of hysteresis increases. For example, the non-linear response is illustrated in a curve 8 in plot 6, which clearly illustrates hysteresis. The increased hysteresis represents the formation of domains in the ferromagnetic cladding layer which produce difficult to control magnetic fields.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved cladding material for magnetoresistive random access memory devices.
It is an object of the present invention to provide a new and improved cladding material which reduces the presence of domains.
It is another object of the present invention to provide a new and improved cladding material which decreases hysteresis and stray fields created by remanent states.
It is another object of the present invention to provide a new and improved cladding material that provides shielding.
To achieve the objects and advantages specified above and others, a multi-layer ferromagnetic cladded conductive interconnect for programming a MRAM device is disclosed which includes a conductive material with a length, a multi-layer ferromagnetic cladding region positioned along the length of the conductive material wherein the multi-layer ferromagnetic cladding region includes N ferromagnetic layers, where N is a whole number greater than or equal to two, and wherein the multi-layer ferromagnetic cladding region further includes at least one spacer layer and wherein the spacer layer is sandwiched therebetween each adjacent ferromagnetic layer. However, in one embodiment, the length of multi-layer ferromagnetic cladding material is formed discontinuous along the length of the MRAM device and positioned on the conductive material in the region proximate to the MRAM device. In the preferred embodiment, the conductive material includes copper and is formed using a standard copper damascene process. Typically, the spacer layer includes a conductive material, an insulating material, or a material that induces anti-ferromagnetic exchange coupling.